Team Fortress is a great action game for the computer. Both my son (15) and I love to play this game and now my 9 year old is showing interest in it as well.
The game is an online multiplayer game. You choose a server and join a team (red or blue), then you pick what type character you will be. Some of the choices are: Heavy (has a machine gun, fists to punch and a shot gun); Spy (has a knife, pistel, electric thing that destroys, discuise case...you can look like anyone you want); Pyro (has the flame thrower, shot gun, axe); Medic (has healing device, and a gun that shoots bad medicine shots, and a bone saw), etc.
The object is to control points on the game or points on the map. Once you can control all the points on the map, you win.
The game, like all war games, is mostly shooting at others. It is fast pace and high action. You use head phones w/ microphone to talk to the other players in the same game. The graphics are cartoon-like. When someone is hit all you see is splashes of red.
I don't let my youngest play this yet, but I would probably rate it less than Mature, maybe PG13.